1. Field of the Invention:
The present invention relates to a process for controlling a photosensitive device consisting of at least one photosensitive point produced by techniques for depositing semiconductor materials such as amorphous silicon. The aim of the invention is to enable images to be taken at random times, the imaging produced by the photosensitive device having as good a quality as possible especially in terms of remanence and stability.
More particularly but not exclusively, the invention relates to the control of such devices used in the detection of radiological images.
2. Discussion of the Background
In order to use these photosensitive devices in the detection of radiological images, a scintillator is inserted between the photosensitive device and the X-ray radiation in order to convert the X-ray radiation into optical radiation in the wavelength band to which the photosensitive point is sensitive. The scintillator material is generally cesium iodide, which is known for its low intrinsic remanence.
A photosensitive point generally comprises a photodiode combined with a switching element having a switching function. The photodiode at least is made from an amorphous semiconducting material. The photosensitive point is mounted between a row conductor and a column conductor. According to requirements, the photosensitive device then comprises a plurality of photosensitive points arranged in a matrix or in a linear array.
The amorphous semiconducting material produces the remanence. This is linked to its amorphous structure which comprises a large number of traps, many more than in crystalline materials. These traps are structural defects which extend throughout the bandgap. They retain charges generated on taking an image. The material stores an image corresponding to a given radiation and restores charges relating to this image on reading the following image or even several following images. The quality of the images thereby suffers.
Another defect affects the quality of the images. The semiconductor components used in such photosensitive devices are not all identical and the photosensitive device has inherent inhomogeneities which result in impaired regions and which vary over time.
To try to obtain a useful image of optimal quality, the useful image is corrected from an image called an “offset image”, also known as a black image, generally taken and stored at the start of an operating cycle. This offset image is the image obtained when the photosensitive device is exposed to a signal of zero intensity and corresponds to a sort of background image. The offset image varies depending on the electrical state of the components of the photosensitive points and of the dispersion of their electrical characteristics.
The useful image is that read when the photosensitive device has been exposed to a useful signal which corresponds to an exposure to X-ray radiation. It encompasses the offset image.
The correction consists in carrying out a subtraction between the useful image and the offset image. This correction is only reliable if the offset image has not varied between the moment where it was taken and the moment where the useful image is taken. It is necessary for the photosensitive points to be in the same electrical state just before taking the offset image and before taking the useful image. In the absence of control, the semiconductor components are continuously searching for an equilibrium state which may be reached in a few hours since the time taken to fill the traps and that taken to empty them of stored charges spreads out over time ranges of between a few microseconds and a few minutes or even a few hours. After this period of time their state may still vary depending on the temperature or on infinitesimal variations of residual irradiation.
Since the offset image is generally taken at the start of the operating cycle of the photosensitive device and since the useful image, actuated at the discretion of the radiologist, is taken randomly as needed, there is no reason for all the semiconductor components to be in the same state at these two times which are separated by a variable time interval.
FIGS. 1a, 1b symbolize of the the fill state of the traps of the components of a photosensitive point of a photosensitive device to which the invention may be applied, over time. The arrows represent imaging cycles. The term “imaging cycle” refers to the sequence consisting of an imaging phase followed by a read phase then by an erasure and reinitialization phase, as explained in Patent Application FR-A-2 760 585. During the imaging phase, the photosensitive points are exposed to a signal to be picked up, whether this signal is at maximum illumination or darkness, during the read phase a read pulse is applied to the addressed row conductors in order to read the amount of charge accumulated on taking the image. During the erasure and reinitialization phase, the photosensitive points are erased, generally optically, and returned to a state in which they are receptive to new imaging.
Between two successive imaging cycles, the photosensitive points are left at rest, but their electrical state changes. It is assumed that the first imaging cycle represented provides the offset image and the others, useful images to be corrected with the offset image.
It can be clearly seen that if the imaging cycles occur randomly, as in FIG. 1a, the electrical states of the photosensitive point being different at the start of the cycle, the useful images corrected with the offset image cannot be reliable.
On the other hand, in FIG. 1b, the imaging cycles occur regularly, for example every five seconds, and at the start of each cycle the electrical state of the photosensitive point is substantially the same.
The offset image has not fluctuated and the correction of a useful image taken during a cycle with the offset image taken during another preceding cycle, is reliable. The major drawback of this operating mode is that it brings many constraints, since the various cycles must follow one another periodically in order to obtain the expected result.
This use is very restrictive and is not compatible with the expectations of radiologists who wish to be able to request images as needed. The present invention proposes to avoid this major drawback while guaranteeing an image of optimum quality.